Adds the ability to hide servers from the main page

[Crossedfall]

Servers explicitly defined as being "hidden" will not show up on the front page, but their join links will still work. Servers without hidden set are assumed to be public.

Example hidden entry:

- id: 'example'
  aliases:
    - 'example'
  name: 'BeeStation Sample Server'
  nickname: 'Example'
  host: 'example.server.com'
  port: 1234
  open: hidden
  hidden: true
  color: '0, 150, 0'
  rules_url: 'https://wiki.beestation13.com/view/Rules'